It has long been felt that a technically viable emissive display technology could compete with the currently dominating technology of liquid crystal displays (LCDs), and OLEDs are presently considered well placed to do so. While the technology of LCDs has various limitations such as low efficiency, poor viewing angles, slow switching speed and narrow temperature ranges, the main advantages of OLEDs are full color, high efficiency, large viewing angles, high switching speed, and low operational temperature. Therefore, organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs) have attracted a tremendous amount of research interests from both academia and industry in the past decade. New light emitting devices based on organic materials are researched and engineered for both display applications and general solid-state lighting. A lot of work is going on in chemistry laboratories to find materials with high luminous quantum efficiency, good color purity and great stability for the application to OLED displays. While some materials meet or exceed some of the requirements for commercial displays, none are believed to meet them all. Efficient and stable red and blue emitters are especially lacking.
Most organic or polymer light emitting diodes emit light through the radiative decay of singlet excitons. As the electron-hole recombination with uncorrelated spins has 75% probability to yield spin symmetric (S=1) state and 25% for spin asymmetric state (S=0), therefore the maximum internal quantum efficiency for the OLEDs and PLEDs using fluorescence as emission mechanism is capped at 25% (photon/electron). Any attempt to further enhance the internal efficiency has to resort to harvesting the triplet excitons in the devices [Baldo, M. A. et al., Nature 1998, 395, 151].
Organometallic complexes containing the third-row transition metal elements such as Os(II), Ir(III) and Pt(II) are crucial materials for this attempt. The strong spin-orbit coupling induced by these heavy metal ions promotes an efficient intersystem crossing from the singlet to the triplet state, which then facilitates high internal quantum efficiencies (ηint) for the OLED devices by using both singlet and triplet excitons. In this regard, numerous attempts have been made to exploit third-row transition metal complexes as dopant emitters for OLED fabrication, among which quite a few Pt(II) and Ir(III) metal complexes have been reported to exhibit highly efficient OLED and PLED device performances.
The design and synthesis of red emitting complexes is intrinsically difficult because their luminescence quantum yield tends to their ionic nature observed in the traditional design involving Os(II) complexes such as [Os(bpy)3][PF6]2, where bpy=2,2′-bipyridine; [Carlson, B. et al., J. Am. Chem. Soc. 2002, 124, 14162]. The OLED devices prepared from [Os(bpy)3][PF6]2 and the related derivatives suffered inferior performances compared with the neutral Pt(II) and Ir(III) counterparts; [(a) Lamansky, S. et al., J. Am. Chem. Soc. 2001, 123, 4304, and (b) Brooks, J. et al., Inorg. Chem. 2002, 41, 3055]. This is, in part, attributed to the lack of strong covalent bonding between the cationic Os(II) emitting complexes and their counter anions within the host matrix. The positively charged Os(II) fragments and their counter anions may undergo significant drifting under high electric field during device operation towards the cathode and the anode, respectively, leading to instability in device performance and a relatively long response time. Accordingly, it is proposed that only the utilization of neutral Os(II) emitting materials, can the goal of practical OLED applications be achieved. In this patent application, we propose the design and preparation of a new series of Os(II) emitting complexes, for which the ligand sphere of the OS(II) atom consists of two anionic chelating ligands such as 3-trifluoromethyl-5-(2-pyridyl)pyrazolate (fppz−), 3-trifluoromethyl-5-(2-pyridyl) triazolate (bptz−), or even (2-pyridyl) tetrazolate (pyN4−) ligand that can neutralize and balance the 2+ charge located at the Os(II) center, and with two donor ligands such as carbonyl, pyridine, bipyridine, arsine, phosphine, and isocyanide ligands to complete the required octahedral coordination arrangement; [Tung, Y.-L. et al., Organometallics 2004, 23, 3745].
Thus, it is an object of the invention to provide Os(II) compound having phosphorescent properties, and uses thereof.